<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span ></span></div>

<script>
    function f(){
        let i = document.querySelector("#i1");
        let s = document.querySelector("span");
        //"5"*"5"   5*5
        //"a"*"a"   NaN*NaN =NaN   不是一个数
        /*isNaN() 判断变量是否是NaN*/
        if (isNaN(i.value)){
            s.innerText="输入有误!"
            return
        }
        //s.innerText = i.value*i.value;
        /*当两个字符串进行-*/

        //s.innerText = i.value*1+i.value*1;//简单粗暴方法
        //parseFloat()  将字符串转成整数或小数
        s.innerText = parseFloat(i.value)+parseFloat(i.value);//

    }
</script>
</body>
</html>